Organic transistor with fluropolymer banked crystallization well

ABSTRACT

A method is provided for fabricating a printed organic thin film transistor (OTFT) with a patterned organic semiconductor using a fluropolymer banked crystallization well. In the case of a bottom gate OTFT, a substrate is provided and a gate electrode is formed overlying the substrate. A gate dielectric is formed overlying the gate electrode, and source (S) and drain (D) electrodes are formed overlying the gate dielectric. A gate dielectric OTFT channel interface region is formed between the S/D electrodes. A well with fluropolymer containment and crystallization banks is then formed, to define an organic semiconductor print area. The well is filled with an organic semiconductor, covering the S/D electrodes and the gate dielectric OTFT channel interface. Then, the organic semiconductor is crystallized. Predominant crystal grain nucleation originates from regions overlying the S/D electrodes. As a result, an organic semiconductor channel is formed, interposed between the S/D electrodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) fabricationand, more particularly, to an organic thin-film transistor (OTFT)fabricated using a fluropolymer that is used as a bank for the purposesof containment and crystallization.

2. Description of the Related Art

As noted in Wikipedia, an organic field-effect transistor (OFET) is atransistor that uses an organic semiconductor in its channel. OTFTs area type of OFET. OTFTs can be prepared either by vacuum evaporation ofsmall molecules, by solution-casting of polymers or small molecules, orby mechanical transfer of a peeled single-crystalline organic layer ontoa substrate. These devices have been developed to realize low-cost,large-area electronic products. OTFTs have been fabricated with variousdevice geometries.

Organic polymers, such as poly(methyl-methacrylate) (PMMA), CYTOP, PVA,polystyrene, parylene, etc., can be used as a dielectric. OFETsemploying numerous aromatic and conjugated materials as the activesemiconducting layer have been reported, including small molecules suchas rubrene, tetracene, pentacene, diindenoperylene, perylenediimides,tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes(especially poly 3-hexylthiophene (P3HT)), polyfluorene,polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene(PPV). These can be deposited via vacuum or solution base methods, thelater being of interest for printed electronics. The newer generation ofsolution processable organic semiconductors consists of blends of highperformance small molecule and polymeric molecules for optimumperformance and uniformity.

FIG. 8 is a plan view photograph of an OSC ink material depositedwithout the benefit of containment banks (prior art). A specific exampleof an organic semiconductor material in a bottom gate OTFT is shown.Since the OSC print does not have any bank control, edge pinning andnucleation of the grains from the edges result, leading to varying grainsize and non-uniform grain growth in the channel. In addition to thecontainment of the OSC ink, it is also important to have some controlover the crystallization in this layer as the solvent evaporates. Lackof OSC containment leads to the inability to form orthogonal geometriesdue to surface tension dynamics. Typically, a deposited OSC ink beginsdrying first at the print area edges and the OSC grains nucleate fromthese edges. As a result, a roughly circular print area is obtained,regardless of the target print geometry, where the crystallizationstarts at the edges and proceeds inwards in the print area. Since moresolvent is driven to the edges, higher grain sizes are obtained at theedges with increasing non-uniformity closer to the center. Suchnon-uniform and unpredictable grain growth is highly undesirable.

In most formulations the solvents being used are volatile enough thatthey start drying immediately after printing, before the anneal step.This drying leads to what is typically termed as a “coffee stain” effectin case of inkjet printing resulting from edge pinning and preferentialdrying at the print edges. This effect causes a solvent flow from theinterior regions of the print area to the edges (convective flow). Thereare some examples in the literature showing that the addition of certainsolvents can reverse this flow (termed Marangoni flow) to some extent.In the case of polymeric systems, which mostly form amorphous films, thecoffee stain effect only leads to variations in the thickness of thefilm from edge to center. However in case of small molecule systems,which are crystallized to form polycrystalline films, there is addedcomplication of the nucleation and grain growth in the film that has tobe controlled. Since the print edges tend to dry first, there is aspontaneous tendency for grain nucleation at the edges. As furthermaterial is drawn from the interior regions of the print area, the graingrowth proceeds towards the center of the print area.

This situation poses two main problems. If the surface tension of thesubstrate forces the formulation into a large nearly circular geometry(as shown), then it is not possible to print very small OSC features. Ifthe volume dispensed is spread over a large area, then the large spreadleads to inadequate volume for large grain growth through the entireprint area—leading to small grains in the middle of the print area wherethe OTFT channel is defined.

In the case of printed small molecule OTFTs, the morphology andpatterning of the organic semiconductor (OSC) layer is a challengingproblem. Two key areas of research involve optimization of the graingrowth in the OTFT channel region and isolation of the channel region ofthe device from the surrounding areas. Organic printed electronics relyon the ability to solution process and/or print each of the TFT layers.Inkjet (IJ) printing is commonly used to print the organic semiconductorlayer. For a successful IJ print of the OSC layer, the solution and thesurface energy must be optimized over a large area to control the extentof the OSC drop spread and uniformity of the drop. This is essential toinsure the consequent morphology is as desired and consistent fromdevice to device. A common way of addressing this issue has been to useso-called bank structures—which is an additional layer that is depositedand patterned in order to create a well structure. The organicsemiconductor material is contained by jetted material only into araised moat region of the coffee stain. The bank material restricts thedrop spread and also helps maintain the solution uniformly over the wellarea. As a result, the process allows for a consistent film thicknessand morphology uniformity with controlled drop drying.

The printing community has been using different methods of fabricatingbank structures. The concept of using fluropolymer materials as an aidin fabrication has been explored to some extent in previous works. Forexample, in U.S. Pat. No. 6,838,361, a printed hydrophobic layer is used(with fluropolymer as example of such layer) to create a separationbetween two printed metal lines, but not necessarily to contain themetal print within certain area. These two metal lines, separated by thefluropolymer, then serve as the source/drain electrodes for a TFT.However, in order for this concept to work, the bank layer must beremoved after the metal layer deposition using a plasma process. Thisremoval process puts limitations on the types of material surfaces thatcan be used. Especially in the case of a bottom gate organic TFT, anyplasma step used to remove the bank layer can have detrimental effect onthe underlying organic gate insulator layer. The plasma step can alsooxidize or otherwise damage the metal S/D interfaces.

Another method using fluropolymers is the deposition of a blanket layer(or dual layers), with patterning using standard lithographictechniques, see US 2007/0193978A1, WO2009077738A1, WO2010020790A1, andWO2009112569A1. The challenge to these approaches is to ensure that thebank material develops cleanly over the organic gate insulator and themetal source drain electrodes, while still not causing any damage tothese structures in the development process.

Gundlach et al., “Contact-Induced Crystallinity for High-PerformanceSoluble Acene-Based Transistors and Circuits”, use a different approachto address the containment issue. In that work they preferentially coatthe S/D electrodes with self-assembled monolayers and then blanket coatan OSC film. Large OSC grain growth nucleates only on these electrodesand then bridges the channel for high performance OTFTs. However, poorgrain growth outside the channel area is used as a means to provide forgood device isolation. This concept works only to a limited extent, withoff currents>nA (nanoamps), and is not reasonable for scaling andapplication in practical products.

It would be advantageous if a banking structure could be deposited usingan inkjet process, to contain OSC deposition, without the requirement oflithographically patterning the bank.

It would be advantageous if the above-mentioned banking structure couldbe left in place after device fabrication.

It would be advantageous if the above-mentioned banking structure aidedin the crystallization of the deposited OSC material.

SUMMARY OF THE INVENTION

Described herein is a simple and effective way of creating bankstructures for small molecule organic semiconductor systems usingfluropolymer ink jet printing. Fluropolymers are ideal for solventorthogonality, creating a large surface energy contrast in order tocontrol the OSC ink spread without contaminating the OSC formulation.The OSC formulations are typically non-polar solvent based and, thus,the fluorinated solvent based systems provide good solventorthogonality. For the same reason, fluropolymers are commonly used as agate insulator for top gate organic TFTs. Also, the ability to formfluropolymer banks via direct inkjet printing, as opposed a depositionand patterning processes, makes it possible to integrate thesestructures within bottom gate process flows without any additionalconcerns regarding its affect on the printed source-drain and gateinsulator layers.

In addition to the containment of the OSC ink, it is also important tohave some control over the OSC crystallization as the solventevaporates. The use of fluropolymer banking around the OSC print area,in tandem with source-drain metal electrodes, reduces or eliminates thetendency of print edge nucleated grain growth, and drives source-drainmediated systematic grain growth in the channel region. It should benoted however that the above-described banking concept is not limited toany particular material systems and is widely applicable provided theformulations and geometries are tuned appropriately.

Accordingly, a method is provided for fabricating an organic thin filmtransistor (OTFT) with a fluropolymer banked crystallization well. Inthe case of a bottom gate OTFT, a substrate is provided and a gateelectrode is formed overlying the substrate. A gate dielectric is formedoverlying the gate electrode, and source (S) and drain (D) electrodesare formed overlying the gate dielectric. A gate dielectric OTFT channelinterface region is formed between the S/D electrodes. A well withfluropolymer'containment and crystallization banks is then formed, todefine an organic semiconductor print area. The well is filled with anorganic semiconductor, covering the S/D electrodes and the gatedielectric OTFT channel interface. Then, the organic semiconductor iscrystallized. Predominant crystal grain nucleation originates fromregions overlying the S/D electrodes. In one aspect the organicsemiconductor crystal grains are formed in the OTFT channel region witha length that extends between the source electrode and the drainelectrode. As a result, an organic semiconductor channel is formed,interposed between the S/D electrodes.

Additional details of the above-described method, a method forfabricating a top gate OTFT, a bottom gate OTFT device, and a top gateOTFT device are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a bottom gate organic thinfilm transistor (OTFT) with a fluropolymer banked crystallization well.

FIG. 2 is a plan view depicting the organic semiconductor layer of FIG.1.

FIG. 3 is a partial cross-sectional view of a top gate OTFT with afluropolymer banked crystallization well.

FIGS. 4A and 4B highlight steps in the process flow of fabricating abottom gate OTFT.

FIG. 5 is a photograph depicting a plan view of the above-described OTFTshowing the OSC film confined within the bank area.

FIG. 6 is a flowchart illustrating a method for fabricating a bottomgate OTFT with a fluropolymer banked crystallization well.

FIG. 7 is a flowchart illustrating a method for fabricating a top gateOTFT with a fluropolymer banked crystallization well.

FIG. 8 is a plan view photograph of an OSC ink material depositedwithout the benefit of containment banks (prior art).

DETAILED DESCRIPTION

FIG. 1 is a partial cross-sectional view of a bottom gate organic thinfilm transistor (OTFT) with a fluropolymer banked crystallization well.The bottom gate OTFT 100 comprises a substrate 102 made from a quartz,glass, plastic, or semiconductor material. A gate electrode 104 overliesthe substrate 102, made from a metal or doped semiconductor material. Agate dielectric 106 overlies the gate electrode 104, typically made froman oxide, nitride, or organic gate insulator. A source (S) electrode 108and a drain (D) electrode 110 overlie the gate dielectric 106, exposinga gate dielectric channel interface region 112 between the S/Delectrodes. Typically, the source 108 and drain 110 electrodes are ametal. A well 114 with fluropolymer containment and crystallizationbanks 116 forms a print area surrounding the gate dielectric channelinterface, and at least a portion of the source 108 and drain 110electrodes. The print area is defined as the gate dielectric channelinterface region 112, the top surface 118 of the source electrode 108,and the top surface 120 of the drain electrode 110. A crystallizedorganic semiconductor layer 122 with a channel 124 overlies the gatedielectric channel interface 112. Note that the drawing is not to scale.

FIG. 2 is a plan view depicting the organic semiconductor layer 122 ofFIG. 1. The crystallized organic semiconductor layer 122 includescrystal grains 200 with lengths 202 that extend between the sourceelectrode 108 and the drain electrode 110. Nucleation origin sites 204overlie the S/D electrodes 108/110, which are depicted in phantom. Thechannel 124 has a first end 206 adjacent the source 108 and a second end208 adjacent the drain 110, with crystal grains 200 extending betweenthe channel first and second ends 206/208, bridging the channel 124.

Returning to FIG. 1, the fluropolymer containment and crystallizationbanks 116 have a height 126 with respect to a gate dielectric channelinterface 112. The crystallized organic semiconductor layer 122 has athickness 128 that can be less than, equal, or greater than the heightof the fluropolymer containment and crystallization banks 116. As shown,the crystallized OSC layer 122 thickness 128 is less than height 126.The fluropolymer containment and crystallization banks 116 are a coffeestain remnant of an inkjet-deposited fluropolymer material, forming aring with a raised moat 130. The moat 130 can be used to collect OSCoverflow.

Returning to FIG. 2, it can be seen that the fluropolymer banks 116 forma rectangular ring with a raised moat 130. As a result of an inkjetdeposition process, the bank structures 116 can be printed such thatthey provide a large coffee stain at the edge of the banks, whichprovide a physical barrier in addition to a surface energy barrier tocontrol the OSC ink spread.

As described in more detail below, the surface energy contrast betweenthe fluropolymer containment and crystallization banks 116 and the gatedielectric channel interface 112 forms a water contact angle on thefluropolymer containment and crystallization banks that is greater thanabout 40. of the water contact angle formed on the gate dielectricchannel interface.

FIG. 3 is a partial cross-sectional view of a top gate OTFT with afluropolymer banked crystallization well. The top gate OTFT 300comprises a substrate 302, with source (S) 304 and drain (D) 306electrodes overlying the substrate 302. A well 308 with fluropolymercontainment and crystallization banks 310 overlies the source 304 anddrain 306 electrodes, defining a print area. A crystallized organicsemiconductor layer 312 at least partially fills the well 308, with achannel 314 interposed between the source 304 and drain 306 electrodes.The print area is defined as the top surface 316 of the source electrode304, the top surface 318 of the drain electrode 306, and a substrateinterface surface 320 interposed between the source and drain. A gatedielectric 322 overlies the channel 314 and a (top) gate electrode 324overlies the gate dielectric 322. Details of the top gate OTFT aresimilar those described above for the bottom gate OTFT, and are notrepeated here in the interest of brevity.

Functional Description

In order to illustrate the above-described devices, an example ispresented of an actual small molecule organic semiconductor based bottomgate organic transistor, as measured in the laboratory. However, itshould be understood that the devices are not limited to the explicitstructure or material systems used in the example. In this particularexample, an organic insulator or OGI, organic semiconductor, Dupontfluropolymer bank (AF1600), and evaporated metal layers are used.

In order to select a particular bank material it is important todetermine the extent of surface energy contrast that can be obtained. Acommon method of performing this task is to contrast the water contactangle on the organic gate insulator material in the channel region withthe water contact angle on the bank region. However, this method doesnot necessarily provide an accurate assessment for the OSC print casesince the base solvent for these inks tend to be non-polar solvents. Soin order to obtain the surface energy contrast of the OSC formulationfor the bank material, the contact angle of the OSC formulation for oneparticular organic gate insulator surface was compared to different bankmaterial candidates. Since any surface treatments can also change thesurface energy drastically, these measurements were performed onsurfaces after appropriate surface treatment steps. The table belowlists different fluropolymers that show a high contrast useful in bankmaterials.

TABLE 1

semiconductor formulation as the contact angle drop. But the table alsoshows that measuring only water contact angle can be misleading. Adifferent fluropolymer system than the AF1600 (“fluropolymer OGI” inTable 1) with the same organic semiconductor solvent is barely 10degrees different than organic gate insulator surface, even though theCA contrast with water is much higher (˜45 degrees).

FIGS. 4A and 4B highlight steps in the process flow of fabricating abottom gate OTFT. FIG. 4A shows the process without using bankstructures and FIG. 4B shows the process flow with bank printintegration.

FIG. 5A is a photograph depicting a plan view of the above-describedOTFT showing the OSC film confined within the bank area. FIG. 5B isprofilometry data. The film of FIG. 5A is highlighted using polarizationcontrast on the optical microscope. The boundary highlights the edges ofthe bank print that are not visible in this contrast.

Significant coffee rings as shown in profilometry data of FIG. 5B.However, the OSC print area has an even higher coffee ring height thanthe banks. The fact that the OSC film produced higher coffee stains thanthe bank print shows the ability of the bank material to confine largequantities of the OSC material effectively. The OSC material iscontained by differences in surface energy as well as banking materialacting as a physical barrier.

The ability to contain large volumes of OSC in the bank area enablesgood uniform grain growth in the OSC film throughout the printed area.In contrast, when unbanked, there is a lack of control over the OSC inkspread, resulting in not only a large area print but also an OSC filmwith highly non-uniform grain growth.

In the case of inkjet printed small molecule organic semiconductorsystems, it is challenging to control the grain growth in the OSC filmas the solvent dries. In most of these formulations, the solvents beingused are volatile enough that they start drying immediately after theprint and even before the anneal step. This leads to what is typicallytermed as a “coffee stain” effect in case of inkjet printing resultingfrom edge pinning and preferential drying at the print edges. Thiseffect causes a solvent flow from the interior regions of the print areato the edges (convective flow). In case of polymeric systems, whichmostly form amorphous films, the coffee stain effect only leads tovariations in the thickness of the film from edge to center. However incase of small molecule systems, which are crystallized to formpolycrystalline films, there is added complication of the nucleation andgrain growth in the film that has to be controlled. Since the printedges tend to dry first, there is a spontaneous tendency for grainnucleation at these edges. As further material is drawn from theinterior regions of the print area, the grain growth proceeds towardsthe center of the print area, as shown in FIG. 8.

This situation poses two main problems. If the surface tension of thesubstrate is such that it forces the formulation into a large nearlycircular geometry, then it is not possible to print very small OSCfeatures. If the volume is widely dispensed, then the large spread leadsto inadequate volume for large grain growth through the entire printarea leading to small grains in the middle of the print area where theOTFT channel is defined. In the case where the OSC material is banked itis still important to be able to control the nucleation and graingrowth. The source-drain electrodes provide preferential nucleationsites. With a larger volume of OSC constrained in the print area, dryingis effectively slowed at the edges, so that SID nucleated grainnucleation is the dominant nucleation and grain growth mechanism. As aresult, the final film morphology is more uniform in the channelregions. This grain growth can be controlled to some extent by tuningthe OTFT channel length (L).

Grain growth is tailored in the OTFT channel by altering the channellength, which is the gap between the printed source-drain electrodes.The assumption is provisionally made that all device dimensions andprocess parameters are identical and only the W (distance between bankwell edges, see FIG. 2) and L are modified. If indeed the grain growthis controlled by the S/D electrodes interfaces then for small channellengths some or all of the grains bridge the entire channel resulting inthe highest mobilities. As L increases, the grains are likely tonucleate at both ends of the channel, grow inwards, and result in agrain boundary in the interior of the channel region. This would resultin degradation of the device mobility. Further an increase in L canresult in a dimensional range where the grain growth in the channel is arandom network of grain boundaries where some grain nucleate in theinterior of channel in addition to in the S/D regions. This result wouldfurther degrade mobility. However, one would also expect the uniformityto improve with increasing L, for the same reasons that the overallmobility would degrade.

The validity of this hypothesis is demonstrated in the measured resultspresented in Table 2. The table indicates that the trend as a functionof L is indeed as expected. Using three devices having a W˜475 um(microns), the microstructure in the channel region is observed as afunction of L. On an average, an increase in grain boundary density isobserved with increasing L. For L ˜23 um, there is a combination ofcases where one or two grains bridge the channel length along differentregions of W. However, the trend towards at least 2 grains along L atdifferent points in W increases for the L ˜60 um. For even higher L ˜105um, there is systematic grain boundary edge in the middle of the channellength along all regions of channel W.

TABLE 2 Mobility (cm²/Vs) 300 475 L\W (um) Average Std. Dev. AverageStd. Dev.  23 0.15 0.04 0.27 0.1   60 0.08 0.03 0.19 0.06 105 0.03 0.020.09 0.04

FIG. 6 is a flowchart illustrating a method for fabricating a bottomgate OTFT with a fluropolymer banked crystallization well. Although themethod is depicted as a sequence of numbered steps for clarity, thenumbering does not necessarily dictate the order of the steps. It shouldbe understood that some of these steps may be skipped, performed inparallel, or performed without the requirement of maintaining a strictorder of sequence. Generally however, the method follows the numericorder of the depicted steps. The method starts at Step 600.

Step 602 provides a substrate. Step 604 forms a gate electrode overlyingthe substrate. Step 606 forms a gate dielectric overlying the gateelectrode. Step 608 forms source (S) and drain (D) electrodes overlyingthe gate dielectric, and a gate dielectric OTFT channel interface regionbetween the S/D electrodes. Step 610 forms a well with fluropolymercontainment and crystallization banks, to define an organicsemiconductor print area. Step 612 fills the well with an organicsemiconductor, covering the S/D electrodes and the gate dielectric OTFTchannel interface. For example, the well can be filled with an organicsemiconductor includes using an inkjet process.

Subsequent to filling the well, Step 614 crystallizes the organicsemiconductor. For example, the device can be furnace or laser annealed.Step 616 forms an organic semiconductor channel interposed between theS/D electrodes. Subsequent to forming the organic semiconductor channel,Step 618 forms a bottom gate transistor with fluropolymer containmentand crystallization banks. In response to defining the organicsemiconductor print area within the fluropolymer containment andcrystallization banks, Step 620 minimizes off-current leakage.

In one aspect, crystallizing the organic semiconductor in Step 614includes predominant crystal grain nucleation originating from regionsoverlying the S/D electrodes. In another aspect, Step 614 forms organicsemiconductor crystal grains in the OTFT channel region with a lengththat extends between the source electrode and the drain electrode.

In yet another aspect, Step 616 forms the organic semiconductor channelwith a length, a width, and an interior region. Then, Step 614 forms twoindependent grain growth fronts from the source and drain electrodesthat grow along the length of the channel and meet in the interiorregion of the channel, forming a systematic single grain boundary frontthat runs through at least a partial width of the channel.

In one aspect, forming the well with fluropolymer containment andcrystallization banks in Step 610 includes forming a predefined printarea. Then, filling the well with the organic semiconductor in Step 612includes forming a predefined area of deposited organic semiconductormaterial between the fluropolymer containment and crystallization banks.

In one aspect, forming the well with fluropolymer containment andcrystallization banks in Step 610 includes using an inkjet printingprocess to form the fluropolymer containment and crystallization banksas coffee-stain rings with a raised moat. Then, filling the well withthe organic semiconductor in Step 612 includes catching overfill organicsemiconductor in the coffee-stain raised moat, in the event ofdeposition misalignment.

FIG. 7 is a flowchart illustrating a method for fabricating a top gateOTFT with a fluropolymer banked crystallization well. The method beginsat Step 700. Step 702 provides a substrate. Step 704 forms source (S)and drain (D) electrodes overlying the substrate, defining a channelinterface surface in the substrate between the source and drainelectrodes. Step 706 forms a well with fluropolymer containment andcrystallization banks surrounding the source electrode, drain electrode,and a substrate interface surface interposed between the source anddrain electrodes, defining an organic semiconductor print area. Step 708fills the well with an organic semiconductor. Step 710 crystallizes theorganic semiconductor. Step 712 forms an OTFT channel in the organicsemiconductor overlying the channel interface surface between the SIDelectrodes. Step 714 forms a gate dielectric overlying the OTFT channel,and Step 716 forms a top gate electrode overlying the gate dielectric.In response to defining the organic semiconductor print area within thefluropolymer containment and crystallization banks, Step 718 minimizesoff-current leakage.

A method has been provided for fabrication OTFT devices using afluropolymer containment banks. Examples of particular structures,process flows, and materials have been presented to illustrate theinvention. However, the invention is not limited to merely theseexamples. Further, materials other than fluropolymer can be used ascontainment structures. Other variations and embodiments of theinvention will occur to those skilled in the art.

1. A method for fabricating a bottom gate organic thin film transistor(OTFT) with a fluropolymer banked crystallization well, the methodcomprising: providing a substrate; forming a gate electrode overlyingthe substrate; forming a gate dielectric overlying the gate electrode;forming source (S) and drain (D) electrodes overlying the gatedielectric, and a gate dielectric OTFT channel interface region betweenthe SID electrodes; forming a well with fluropolymer containment andcrystallization banks, to define an organic semiconductor print area;and, filling the well with an organic semiconductor, covering the S/Delectrodes and the gate dielectric OTFT channel interface.
 2. The methodof claim 1 further comprising: subsequent to filling the well,crystallizing the organic semiconductor; and, forming an organicsemiconductor channel interposed between the S/D electrodes.
 3. Themethod of claim 2 wherein crystallizing the organic semiconductorincludes predominant crystal grain nucleation originating from regionsoverlying the S/D electrodes.
 4. The method of claim 2 whereincrystallizing the organic semiconductor includes forming organicsemiconductor crystal grains in the OTFT channel region with a lengththat extends between the source electrode and the drain electrode. 5.The method of claim 2 wherein forming the organic semiconductor channelincludes forming the channel with a length, a width, and an interiorregion; and, wherein crystallizing the organic semiconductor includesforming two independent grain growth fronts from the source and drainelectrodes that grow along the length of the channel and meet in theinterior region of the channel, forming a systematic single grainboundary front that runs through at least a partial width of thechannel.
 6. The method of claim 2 further comprising: subsequent toforming the organic semiconductor channel, forming a bottom gatetransistor with fluropolymer containment and crystallization banks; and,in response to defining the organic semiconductor print area within thefluropolymer containment and crystallization banks, minimizingoff-current leakage.
 7. The method of claim 1 wherein filling the wellwith an organic semiconductor includes using an inkjet process to fillthe well with organic semiconductor.
 8. The method of claim 1 whereinforming the well with fluropolymer containment and crystallizationbanks, to define the organic semiconductor print area includes forming apredefined print area; and, wherein filling the well with the organicsemiconductor includes forming a predefined area of deposited organicsemiconductor material between the fluropolymer containment andcrystallization banks.
 9. The method of claim 1 wherein forming the wellwith fluropolymer containment and crystallization banks includes usingan inkjet printing process to form the fluropolymer containment andcrystallization banks as coffee-stain rings with a raised moat.
 10. Themethod of claim 9 wherein filling the well with the organicsemiconductor includes catching overfill organic semiconductor in thecoffee-stain raised moat, in the event of deposition misalignment.
 11. Amethod for fabricating a top gate organic thin film transistor (OTFT)with a fluropolymer banked crystallization well, the method comprising:providing a substrate; forming source (S) and drain (D) electrodesoverlying the substrate, defining a channel interface surface in thesubstrate between the source and drain electrodes; forming a well withfluropolymer containment and crystallization banks surrounding thesource electrode, drain electrode, and a substrate interface surfaceinterposed between the source and drain electrodes, defining an organicsemiconductor print area; filling the well with an organicsemiconductor; crystallizing the organic semiconductor; forming an OTFTchannel in the organic semiconductor overlying the channel interfacesurface between the S/D electrodes; forming a gate dielectric overlyingthe OTFT channel; and, forming a top gate electrode overlying the gatedielectric.
 12. The method of claim 11 further comprising: in responseto defining the organic semiconductor print area within the fluropolymercontainment and crystallization banks, minimizing off-current leakage.